Order of nucleophilicity is

Explanation

To determine the order of nucleophilicity among the given species, we need to analyze the factors that influence nucleophilicity, such as the electronegativity of the atom bearing the lone pair and the inductive effects of substituents.

### Step-by-step Solution:

1. **Understanding Nucleophilicity**:
Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophile. The more electron-dense a nucleophile is, the stronger it is as a nucleophile.

2. **Comparing CH3O- and C2H5O-**:
- **CH3O- (methoxide)** has a methyl group (CH3) which is weakly electron-donating.
- **C2H5O- (ethoxide)** has an ethyl group (C2H5) which is a stronger electron-donating group due to its larger size and greater +I (inductive) effect.
- Therefore, CH3O- < C2H5O- in terms of nucleophilicity.

3. **Comparing C2H5O- and C2H5S-**:
- **C2H5O- (ethoxide)** has oxygen, which is more electronegative than sulfur.
- **C2H5S- (ethanethiolate)** has sulfur, which is less electronegative and can donate its electrons more readily.
- Thus, C2H5S- > C2H5O- in terms of nucleophilicity.

4. **Comparing CH3O- and CH3S-**:
- **CH3O- (methoxide)** again has oxygen, while **CH3S- (methanethiolate)** has sulfur.
- Similar to the previous comparison, sulfur's lower electronegativity makes CH3S- a stronger nucleophile than CH3O-.
- Therefore, CH3O- < CH3S- in terms of nucleophilicity.

5. **Final Order of Nucleophilicity**:
- From the comparisons made, we can summarize the order as follows:
- CH3O- < C2H5O-
- C2H5O- < C2H5S-
- CH3O- < CH3S-

Thus, the overall order of nucleophilicity is:
- CH3O- < C2H5O- < C2H5S- < CH3S-